Box revealed in her post that she was expecting her child after undergoing IVF with an anonymous sperm donor.

She heaped praise on her "amazing" IVF doctor, who helped her achieve her dream.

"After a series of egg retrievals and freezing, last year I started the process to become pregnant via an anonymous donor," she said.

Box gave birth to Trixie, whose father is former Ironman Grant Kenny, in 2013.

The pair split after a brief romance in 2012 but are reportedly on good terms, with Kenny regularly visiting his daughter in Melbourne.

The TV personality said she was "blessed" to be having a "much-wanted sibling" for her daughter.

"I know the value of this miracle because I have seen first hand the heartbreak, tears and sacrifices so many women and couples suffer on their IVF journeys and my heart goes out to those who are facing that pain and struggle," Box wrote in her emotional post.